Fast-reacting polyurea coatings cannot be applied directly to concrete surfaces which contain defects called “bug holes”. Bug holes are small regular or irregular cavities resulting from trapped air bubbles in the surface of formed concrete during its installation. Such bug hole defects may be found in both new and old concrete. When the bug hole is overcoated with a polyurea coating during repair processes, the gas trapped under the coating is heated by the exothermic polyurea coating. The expanding gas causes the coating to bubble producing a mushroom shape or a hole in the coating (if the bubble breaks). This hole must be patched and recoated.
Workers in the art have tried many approaches to repair bug holes before coating with polyurea. Epoxy primers, polyurethane/castor oil primers and latex primers have all been tried.
For example, U.S. Pat. Nos. 5,759,695 and 5,962,144, both issued to Primeaux, II, disclose polyurea elastomer systems which are said to have improved adhesion to a substrate with the use of a primer that is applied first. The primer is made of a hydrophobic, primary hydroxyl-containing compound, for example, castor oil, and an isocyanate. Primeaux, II states that it is not necessary to wait for the primer to cure prior to applying the polyurea elastomer system. The primer system is said to be particularly useful on wet substrates, where the primer penetrates the substrate prior to polyurea elastomer spraying.
Mishra, et al., in U.S. Pat. No. 6,376,579, report an epoxy composition said to be especially designed for use as a primer for sealing and filling small pores in concrete. Mishra, et al. state that their composition is a low temperature curing, sag-resistant epoxy primer, which provides good adhesion to concrete and to a polyurea coating applied to the primed concrete. Their epoxy primer is said to be curable at a temperature below 40° F. and is an admixture of two parts, Component A and Component B; in which Component A is a crystallization resistant reactive epoxy resin and Component B is an amine curing agent. The resulting primer bonds to concrete at 200 psi or greater, when measured by ASTM D 4541.
U.S. Pat. No. 6,875,500, issued to McGrath, Jr., discloses a flexible flooring system for floors exhibiting surface movement, and/or for cushioning hard floors, which includes an epoxy or polyurethane primer, a two-component ambient temperature cured polyurethane membrane substrate base atop the primer of a urethane grade castor oil, polyether based polyol, a tin catalyst and a thickening agent, along with a polymeric isocyanate activator, and a polyurethane or polyurea, aromatic or aliphatic coating atop the substrate base. The flooring system of McGrath, Jr. is said to provide a very tough, but flexibly soft non-cracking overlay which moves with the floor.
Primers have been shown to reduce the surface porosity and reduce the blistering of a subsequently applied coating. However, a thin coating of a primer usually does not solve the problem of bug holes, because although the primer coats the holes and reduces the porosity of the substrate, deep holes usually remain in the substrate. Thus, the application of a coating still results in a trapped pocket of gas in the deeper holes.
Another option is to use a sealant made with an isocyanate-terminated prepolymer to patch bug holes and fill seams in the concrete. However, if such a sealant is applied to wet concrete or used in high humidity conditions, the prepolymer likely will react with water. The isocyanate/water reaction produces carbon dioxide gas as a side product. Therefore, bubbling or foaming of the sealant may occur.
Thus, a need exists in the art for a process for coating or sealing concrete in restoration and repair processes. The process should be capable of repairing bug holes and filling seams in wet concrete and/or in high humidity environments.